1. Field of the Invention
The present invention generally relates to methods and apparatuses for reproducing video and audio, and particularly relates to a method and a device for reproducing video and audio that is capable of a special form of reproduction such as pause or slow reproduction of video and audio.
2. Description of the Related Art
Data of video and audio used in the DVD (digital versatile disc) or the like are compressed and coded by the MPEG or a relevant scheme (Dolby Digital, DTS, or the like), and, then, are divided into packets, each of which is provided with attached time information (PTS: presentation time stamp) for the synchronization purpose. Each packet is further provided with common time information (SCR: system clock reference) to be recorded as multiplexed coded data.
FIG. 1 is an illustrative drawing for explaining data coding and multiplexing.
Video information is coded by the unit of one field or one frame that is a basis of presentation, and audio information is also coded by the unit of one frame. One frame of audio information generally differs in length from one frame of video information. PTS is attached to each unit for coding. When one packet includes a plurality of audio frames, however, PTS is attached only to the first frame. In the example of FIG. 1, therefore, audio data frames 1, 3, and 5 each have PTS attached thereto, whereas audio data frames 2, 4, and 6 are not provided with attached PTS. These data frames are thus lacking in time information.
When data is reproduced, the multiplexed video/audio codes are separated and decoded, and, at the same time, a system clock is generated based on SCR, i.e., the system time reference. By using the system time clock as a reference, decoded video data and audio data are output at timings specified by the attached PTSs.
In general, video information is presented on a display device such as a TV set, so that timings of presentation are restricted by synchronizing clock timings of the device. For example, therefore, the system time clock is counted based on the synchronizing clock timing, and pictures are presented at such timings as the count corresponds to the PTSs. Audio data has a frame length different from that of video frames, so that the audio data is not reproduced at the same timing as the video data. The audio data is reproduced by determining its own output timings based on PTSs of the audio data. With respect to an audio data frame that has no PTS attached thereto, audio data is output continuously without any gap following the previous frame, thereby maintaining synchronization with the video presentation.
A special form of reproduction such as pause or slow reproduction may be performed. In the case of pause reproduction, a picture corresponding to the paused timing is repeatedly presented for paused video presentation, and audio data is suspended. In the case of slow reproduction, short duration of pause and play is repeated as many times as necessary.
FIG. 2 is an illustrative drawing for explaining pausing of reproduction and resuming of reproduction.
FIG. 2 shows a case in which the user instructs pause during presentation of a second video picture. In response to the user's pause instruction, the video picture 2 is displayed again after the completion of current presentation of the video picture 2. As long as the pause state continues, the video picture 2 is repeatedly displayed. When the user instructs the pause, a third audio frame is being reproduced. Accordingly, reproduction of audio data is suspended after the end of reproduction of the audio frame 3.
When the user's pause instruction comes to an end, a next video picture 3 is presented after waiting for completion of the video picture 2 that is currently displayed as part of the pause operation. As shown at the top of FIG. 2, it is desirable to resume reproduction of audio data in correct synchronization with the video data halfway through the presentation of the resumed video picture 3. If the audio data frame 4 is provided with a PTS, reproduction of the audio data frame 4 can be properly started at the timing indicated by the PTS.
If the audio data frame 4 is not provided with a PTS, however, it is uncertain when the reproduction of the audio data frame 4 should be resumed. In such a case, the audio data frame 4 having no PTS may be discarded as shown in Conventional Audio Reproduction Method 1 of FIG. 2, and the next audio data frame 5 having a PTS may be presented at the timing specified by this PTS. When this is done, however, audio for the audio data frame 4 is lost, resulting in silent reproduction of video information.
Alternatively, as shown in Conventional Audio Reproduction Method 2 of FIG. 2, the audio data frame 4 having no PTS may be reproduced concurrently with the start of reproduction of video information, with timing of audio reproduction being adjusted according to the PTS of the next data frame 5 at the time of reproduction of the audio data frame 5 having the PTS attached thereto. In this case, however, noise is audible at the gap between the audio data frame 4 and the audio data frame 5.
Accordingly, there is a need for a scheme that can properly resume reproduction of audio information after completing a pause of video and audio information.
Another drawback of the related art relates to a special form of reproduction for reproduction of a movie or the like on the TV set. Movies have 24 frames per second, whereas the NTSC standard of TV sets includes 30 frames per second, for example. Here, each TV frame is comprised of two fields. When movies are presented on the TV set, there is a need to allocate 60 TV fields per second to 24 movie frames per second. To this end, some movie frames each have two TV fields allocated thereto, and the other movie frames each have three TV fields allocated thereto, thereby establishing proper correspondences between 60 TV fields and 24 movie frames.
When a special form of reproduction is carried out with respect to such video information as configured above, a problem may be encountered since movie frames each corresponding to two fields are mixed with movie frames each corresponding to three fields. As previously described, slow reproduction is performed by repeating a short duration of pause followed by a short duration of play. During the pause operation, a frame having three fields attached thereto may be displayed. If the slow reproduction is designed to repeat pause operation by treating frames as simply having two fields, therefore, a timing displacement is bound to arise.
FIG. 3 is an illustrative drawing for explaining slow reproduction when two field frames and three field frames are mixed together.
As shown in FIG. 3, a video signal includes a first frame comprised of two fields 1T and 1B, a second frame comprised of three fields 2T, 2B, and 2T, a third frame comprised of two fields 3B and 3T, a fourth frame comprised of three fields 4B, 4T, and 4B, and a fifth frame comprised of two fields 5T and 5B.
When this video signal is presented in slow reproduction, the first frame comprised of the two fields 1T and 1B is displayed first, and, then, these two fields 1T and 1B are repeated again in the same manner as during pause operation. Then, the second frame comprised of the three fields 2T, 2B, and 2T are displayed, followed by the first two fields 2T and 2B of these three fields 2T, 2B, and 2T being displayed again. Further, the third frame comprised of the two fields 3B and 3T is displayed, and, then, these two fields 3B and 3T are repeated again in the same manner as during pause operation. When this is done, the time length having passed for the slow reproduction comes short of the time length 2T that is double the time length T required for normal reproduction of the first through third frames, stopping at the end of the time length TS. This is because the second frame comprised of the three fields 2T, 2B, and 2T is simply treated as a frame having two fields, and only the two fields 2T and 2B are repeated when reproduced as part of the pause operation.
In this case, therefore, the user may assume half-as-slow reproduction, yet the passage of time does not correspond to 2T that is two times as long as T. The actual passage of time is TS, which means a failure to achieve half-as-slow reproduction.
In this manner, even when two-field frames and three-field frames are mixed together, the related-art method repeats reproduction for pause operation by treating one frame as simply having two fields. As a result, actual reproduction time ends up differing from the supposed time length.
Accordingly, there is a need for a scheme that performs pause operation according to the actual number of fields by discriminating three-field frames from two-field frames when reproduction of video and audio is paused.
In summary, there is a need for a scheme that can properly resume reproduction of audio information after completing a pause of video and audio information.
Further, there is a need for a scheme that performs pause operation according to the actual number of fields by discriminating three-field frames from two-field frames when reproduction of video and audio is paused.